I just bought a 24-105L I notice a sound when pressing the shutter to AF. It is difficult to describe the sound but I don't hear this kind of sound on my 70-300L Now I know the 70-300L is much newer and may have a more advanced mechanism.

I am just not used to this kind of sound and wonder if it is normal. I just like to hear your experiences before going back to the store.

Are you're hearing the IS motor kicking in when you half-press the shutter? Turn off IS to check. A small sound at start/stop is normal for most IS lenses, volume varies (with some lenses, you need to almost have your ear against the barrel, others like the old 300/4 IS start with a loud clunk.

From your questions and explanation I have somehow the impression that my lens behaves normal. Just because I don't have this on my 70-300L I was worried.

Here is my little test:When I half-press the shutter, the IS motor is starting. It makes a sound like a scratch - so less noisy than a clunck, The scratch is followed by a gentle sound of a circular saw. When I release the shutter again I hear the scratch.